Ideally, a wind turbine blade of the airfoil type is shaped similarly to the profile of an aeroplane wing. On a wind turbine rotor the chord plane width of the blade as well as the first derivative thereof increase continuously with decreasing distance from the hub.
This results in the blade ideally being comparatively wide in the vicinity of the hub. This again results in problems when having to mount the blade to the hub, and, moreover, this causes great loads during operation of the blade, such as storm loads, due to the large surface area of the blade.
Therefore, over the years, construction of blades has developed towards a shape, where the blade consists of a root region closest to the hub, an airfoil region comprising a lift-generating profile furthest away from the hub and a transition region between the root region and the airfoil region. The airfoil region has an ideal or almost ideal blade shape with respect to generating lift, whereas the root region has a substantially circular cross-section, which reduces the storm loads and makes it easy and safe to mount the blade to the hub. Preferably, the root region diameter is constant along the entire root region. Due to the circular cross-section, the root region does not contribute to the energy production of the wind turbine and, in fact, lowers this a little because of drag. As it is suggested by the name, the transition region has a shape gradually changing from the circular shape of the root region to the airfoil profile of the airfoil region. Typically, the width of the blade in the transition region increases substantially linearly with increasing distance from the hub.
As for instance wind turbine blades for wind turbines have become bigger and bigger in the course of time, and they may now be more than 60 meters long, the demand for optimised aerodynamic performance has increased. The wind turbine blades are designed to have an operational lifetime of at least 20 years. Therefore, even small changes to the overall performance of the wind turbine blades may accumulate over the lifetime of a wind turbine blade to a high increase in financial gains, which surpasses the additional manufacturing costs relating to such changes. For many years, the focus areas for research have been directed towards improving the airfoil region of the blade, but during the recent few years more and more focus has been directed towards also improving the aerodynamic performance of the root and transition regions of the blade.